1. Field of the Invention
This invention relates generally to measuring distance, and more particularly to determining the distance during a regatta between a sailboat and a race course marker buoy.
2. Description of the Related Art
It is very important for the crew of a sailboat in a regatta to know how many boat lengths a boat is away from a race course marker buoy. The two boat length distance from the buoy is the point at which the rules change. Thus, as boats in a regatta converge on a buoy, it is important for those boats to know when each boat crosses its two boat length distance from the buoy.
If the boats on the water were cars driving around a specific marker cone placed on the ground, it would be trivial to draw on the ground a circle around the cone, where the circle would have a radius equal to two car lengths. If the cars were of different lengths, the circles could be drawn in different colors, different textures, or any of a number of ways to distinguish the different circles. All cars would then be able to see when one of the cars crosses into that car's circle. Because sailboats move through water, vice cars driving on ground, there is no directly equivalent trivial way to draw a circle around a buoy on the water for all sailors to see, and through or over which boats can pass unobstructed.
What is need is a device which indicates to the crew of its boat and to others when that boat has reached a certain distance away from the buoy.
A number of methods have been developed to measure distances when it is not practical to mark them on the ground or water. Some involve the use of multiple receivers or transmitters, or measurements taken at multiple points, in order to do some form of triangulation. Triangulation methods to determine distance are not of concern here.
Other methods involve the use of time, measuring how long in time it takes a signal to travel the distance being measured, then using that time measurement, along with knowledge of signal's propagation speed through the transmission medium, to convert the time measurement into a distance measurement. The prototypical time method is radar: a source sends out a signal and times how long it takes for the reflection to return. By knowing the rate at which the signal travels, the time of flight can be converted to a distance. Traditional radar uses electromagnetic signals which travel at the speed of light, and divides the time of flight by two since the radar signal transits the distance twice; out and back. Traditional radar is a very good solution for measuring distance when it cannot otherwise be marked on the ground, especially large distances on the order of miles or farther.
For measuring relatively short distances, on the order of feet or inches or less, adaptations have been made to traditional radar. For example, because electromagnetic signals travel at the speed of light, special considerations are needed when using such a fast medium to measure short distances. While distance resolutions on the order of feet, or less, are achievable using electromagnetic based radar, it comes at added costs of size or weight or processing power or complexity or some combination there of.
When the operational environment is not conducive for expansion of size, weight, processing power or complexity, one alternative may be to switch from electromagnetic signals to acoustic signals. This method is typified by sonar and embodied in the acoustic tape measure. The components for generating sound and receiving the reflected sound can be small, light weight, consume little power, inexpensive and are readily available in the commercial market place. Further, because the speed of sound through air is many orders of magnitude slower than the speed of an electromagnetic signal, getting plus or minus one foot resolution, or better, is easily achievable. Handheld battery powered acoustic tape measures are relatively inexpensive commercially available products.
The next evolutionary step in measuring distance is to send an electromagnetic signal in conjunction with an acoustic signal. U.S. Pat. No. 4,136,394, a golf yardage indicator system, is such a system: it transmits an electromagnetic pulse in one direction and a return acoustic pulse in the other direction. In contrast, U.S. Pat. No. 4,055,830, a sonic measuring system, U.S. Pat. No. 4,234,942, an apparatus for measuring the length of pipe and other hollow members, U.S. Pat. No. 5,191,328, a trailer hitching aid, and U.S. Pat. No. 6,404,703, a method and apparatus for distance measurement, each send out both an electromagnetic signal and an acoustic signal in the same direction. In all cases, the speed of the electromagnetic signal is so much greater than the speed of the acoustic signal that the time for the electromagnetic signal can be considered to be zero and the time for the acoustic signal can be converted into a distance measurement.
These systems are each able to utilize a benign operating environment to their own advantage. The golf yardage indicator system does not need to operate for multiple simultaneous measurements because golfers play in a genteel, cooperative manner. The pipe length measuring method is able to operate in the stable environment inside of the pipe. The trailer hitching aid only need operate over distances equal to little more than the length of a trailer. The distance measurement method of U.S. Pat. No. 6,404,703 adds processing and multiple signals to enhance signal detection, along with specifically using the directional nature of ultrasonic signals to provide bearing information.
Each of those U.S. Patents, implicitly or explicitly, also touches on the issue of calibrating for the speed of sound. U.S. Pat. No. 4,055,830 mentions how the speed of sound through air is related to the air temperature. In general, the speed of sound through air can vary from 331 meters per second (m/s) at a temperature of 0 degrees Celsius and 0% relative humidity to 351 m/s at 30 degrees Celsius and 100% relative humidity. This is a 6 percent variation. In contrast, the speed of sound through water can vary from 1402 m/s at 0 degrees Celsius and 0 salinity to 1551 m/s at 30 degrees Celsius and 40 on the practical salinity scale (a 40 on the practical salinity scale would be salty sea water). This is an 11 percent variation. (Pressure does effect the speed of sound in air and water. The just mentioned speeds are for air pressure at sea level and water pressure just below the surface; which are the conditions assumed for this discussion.) Thus the need for accurate and precise calibration in air is almost less than half of what it is in water. To the degree the issue is only relative distance (i.e., getting closer or farther) vice the actual distance measurement, calibrating the speed of sound through its environment may be unnecessary.
While the distance measuring methods discussed heretofore have not directly addressed the issue of measuring the distance between a race course marker buoy and a sailboat during a regatta, that does not mean this problem has not been previously addressed. Some media coverage of the America's Cup regatta (currently one of the most famous, premier sailboat races in the world) in recent years has contained animated graphic presentations which show the boats on the water, in relation to each other as well as in relation to race course marker buoys. These graphics show the two boat length circle around the marker buoys. Such graphic displays are the direct visual equivalent of being able to draw a circle on the water, just like the opening analogy above of drawing circles on the ground for cars going around a cone.
The system used with the America's Cup utilizes position data from sources like the Global Positioning System (GPS). GPS by itself does not report plus or minus one foot, or less, resolution. One solution for improving GPS resolution is to use Differential GPS (DGPS). Another solution is to use an Inertial Navigation Systems (INS). Yet another solution is to do statistical averaging or other mathematical manipulation of the reported data. Whether using GPS, DGPS, INS, statistics, or some other method to determine and report each boat's position to the graphics generator, each buoy also needs to have its position determined and reported to the graphics generator. This can be done for example by having a GPS or DGPS receiver on the buoy, or the crew setting the buoy can have such a receiver and use it when the buoy is set. After the graphic generator has all the information it needs and does its job, the graphic presentation then has to be distributed. For an animated graphic display, the whole process has to be done on a continuing basis. Such a system however requires all boats and buoys to have some capability for of determining their positions, radios for communicating with the graphic equipment, expensive and complicated graphic processor generation equipment, and the communications path for distributing the information generated by the processor.